1. Field of the Invention
The present invention relates generally to a friction-roller-type continuously variable transmission for automobiles or the like, and more specifically to a speed ratio control device for such transmissions.
2. Description of the Related Art
A friction-roller-type continuously variable transmission for automobiles or the like is known, e.g., from JP-A-4-78,365 and JP-A-4-78,366. The transmission as known from these patent documents includes friction rollers which are arranged in a toroidal groove formed between input and output disks and rotatably supported by rotatable and axially movable roller support members having respective rotational axes which are perpendicular to the common axis of the input and output disks. The speed ratio of the transmission between the input and output disks is determined in accordance with the axial position of the friction rollers. Thus, hydraulic cylinder units are provided to axially move the roller support members and hence friction rollers relative to the input and output disks. The transmission further includes a forward drive/reverse drive switching mechanism for effecting a switchover between a forward drive mode and a reverse drive mode of the transmission. For controlling the speed ratio of the transmission, there is provided a speed ratio control device which includes a forward drive speed ratio control valve and a reverse drive speed ratio instruction valve. The forward drive speed ratio control valve is operative when the switching mechanism assumes the forward drive position, and serves to control the speed ratio in the forward drive mode. The reverse drive speed ratio instruction valve is operative when the switching mechanism assumes the reverse drive position, and serves to provide a constant speed ratio in the reverse drive mode.
In order to ensure a stabilized maintenance of the instructed speed ratio, the forward drive speed ratio control valve is provided with a precession cam and a link for a performing feedback of the angular displacement of one of the roller support members to the forward drive speed ratio control valve. Also, in order to achieve a predetermined constant reverse drive speed ratio, the reverse drive speed ratio instruction valve is provided with another precession cam and another link for performing a feedback of the angular displacement of the other roller support member to the reverse drive speed ratio instruction valve.
The above-mentioned conventional arrangement of speed ratio control device for the friction-roller-type continuously variable transmissions proved to provide a satisfactory control of the forward drive speed ratio and the reverse drive speed ratio. However, the provision of a set of precession cam and link associated with one roller support member, and another set of precession cam and link associated with another roller support member, often makes it difficult to reduce the number of components and cost of the transmission. While the transmission includes relatively large components, such as solenoid valves, step motors, and the like, these components cannot be arranged within movable ranges of the two precession cams and two links. Such components have to be arranged outside the movable ranges of the cams and the links, so that the transmission as a whole becomes bulky and heavy in weight. Moreover, the limitation in the arrangement of the components makes it often difficult to get an easy access thereto as required for assembly or maintenance of the transmission. As the case may be, the assembly or maintenance of the transmission cannot be performed without removing the entire control valves.